1. Field of the Invention
The invention relates to an electrostatic discharge protection structure, and more particularly, to an electrostatic discharge protection structure capable of enduring negative voltage.
2. Description of the Prior Art
Electrostatic discharge is an effect due to electrical charges in an object flowing to another object through a discharging path when two electrically charged objects contact or short to each other. The electrostatic discharge can generate huge currents in a very short time and can damage an integrated circuit. Since the human body, machines used to package integrated circuits, and instruments for testing integrated circuits are all common charged bodies, the static electric charges of a charged body may discharge to a chip and cause irreversible harm to the chip once the charged body contacts with the chip. Therefore, an electrostatic discharge protection device is designed to provide a low resistance discharge path for the huge currents induced during electrostatic discharge to pass by and to protect the integrated circuit from being blown out.
To avoid the case that the electrostatic discharge protection device may be accidentally turned on under a normal operation situation and disturb the operation of the protected chip, the holding voltage of the electrostatic discharge protection device should be outside of the operational voltage range of the chip. For example, if the operational voltage range of a chip is between 1.2V and 1.5V, then the holding voltage of the electrostatic discharge protection device should be greater than 1.5V so that the electrostatic discharge protection device will not be turned on when an input voltage of the chip is 1.5V. However, in some applications, the operational voltage range of a chip may include negative voltage. For example, the operational voltage range may be −3V to 3V. In some cases, the upper limit and the lower limit of the operational voltage range may even be unsymmetrical, such as an operational voltage range between −10V to 20V. However, the common electrostatic discharge protection device may only protect the chip from being damaged by currents in the same direction and within limited ranges of voltage. Therefore, the design of the electrostatic discharge protection device is rather inflexible and may even cause inconvenience when producing the electrostatic discharge protection device.